1. Field of the Invention
The present invention relates to a pressure control valve for keeping the interior of a pressure circuit at a specified setting pressure.
2. Description of the Related Art
As a pressure control valve for keeping the interior of a pressure circuit at a specified setting pressure, a direct-operated relief valve 2, for example, is known which is illustrated in FIG. 6.
The direct-operated relief valve 2 keeps a relief port 3, which is open to the pressure circuit, closed with a valve plug 6 which is pressed by a pressure adjusting spring 4 against the relief port 3. When the pressure in the pressure circuit rises above a specified setting pressure, the valve plug 6 is pushed up and the relief port 3 communicates with the pressure circuit and part of the fluid flows out of the pressure circuit into the drain port, so that the pressure circuit can be held at a specified setting pressure. Note that the setting pressure is adjusted by changing the pressure of the spring by turning the adjust screw 8.
In the direct-operated relief valve 2, the pressure adjusting spring 4 also presses the valve plug 6 and therefore it is necessary to use a strong spring for the pressure adjusting spring 4 when the direct-operated relief valve 2 is used in a pressure circuit at high pressure. However, the use of a large, strong spring makes the relief valve itself large and there is a limit to the strength of the spring used. For this reason, normally, a pilot-operated relief valve 10 shown in FIG. 7 is adopted in pressure circuits for high pressures.
The pilot-operated relief valve 10 is structured such that the relief port 3 communicates through a throttle 12 with an upper chamber 14a of a main valve 14 and also with a direct-operated relief valve 18 that constitutes a pilot section as illustrated in FIG. 7. When the pressure of the pressure circuit rises above a specified setting pressure and surpasses the pressure of the spring 18b, the valve plug 18a goes down and the direct-operated relief valve 18 opens. Consequently, the upper chamber 14a communicates with the drain port 5, the pressure of the upper chamber 14a decreases, with the result that the main valve 14 opens and the fluid of the pressure circuit flows to the side of the drain port 5. Thus, the pressure circuit at high pressure can be kept at the specified setting pressure.
For use as an alternate pilot section to replace the direct-operated relief valve 18 of the pilot-operated relief valve 10 shown in FIG. 7, if a proportional electromagnetic pressure control section for controlling the pressure of the upper chamber 14a by an electric pilot signal is incorporated, it is possible to implement automatic control of the setting pressure of the pressure circuit within a specified range.
However, if the proportional electromagnetic pressure control section mentioned above is used, there is a possibility that the resultant pressure control valve is a type of a complicated internal structure, which increases production cost and poses a problem of safety, such as a fire caused by the use of electricity.
The present invention has been made with the above problem in mind and has as its object to provide a pressure control valve capable of stepless automatic control of the setting value of a pressure circuit, which is made possible by the adoption of a pilot-operated simple-structure without use of electric signals.